The present invention relates to a magnetic disk apparatus having an off-track correcting function to correct an off-track amount that is peculiar to a data head upon positioning of the head and, more particularly, to a magnetic disk apparatus for measuring an off-track amount during the operation of the apparatus and for forming a correction table.
In the present magnetic disk apparatus, the improvement of the performance such as large memory capacity, high data transfer speed, or the like is more and more progressed. The miniaturization of the magnetic disk apparatus is also more and more progressed to meet requirements from installation circumstances such as reduction of a space of a computer center or decrease in electric power consumption and to improve an access time. A track density (TPI) in the radial direction per inch of the magnetic disk and a recording density (BPI) in the circumferential direction per inch are also more and more increased.
When the recording density is raised as mentioned above, to prevent the occurrence of an off-track due to a temperature change or the like, it is important to measure an off-track amount for a reference position and to execute an off-track correction upon positioning of the head.
Hitherto, in the case where the track density rises because a track interval is narrowed in the magnetic disk apparatus, according to the conventional head positioning control by only servo information of the servo surface, the position of a data head is deviated from the position of a servo head due to an environmental change in temperature of the apparatus, an external force which is applied to a head actuator, or the like. Even when the seeking operation is performed, therefore, there is a fear that an off-track occurs between the present track and a target track and the reading and writing operations cannot be stably executed. On the other hand, inconveniences such that the seeking operation becomes unstable and a settling time becomes long occur.
To eliminate the above problems, there is performed an off-track correction such that servo information is recorded into the data surface of a disk medium and an off-track amount is previously measured every data head and, when a control mode is switched to a position control after completion of the seeking operation, the head is moved in such a direction as to eliminate the measured off-track amount.
That is, when a disk control unit receives a command chain from a higher-order CPU, the disk control unit executes a speed control such that a seek command including a cylinder address is supplied to the magnetic disk unit and the head is moved to a target cylinder position by a magnetic disk apparatus.
In the speed control, an acceleration current is first supplied to a voice coil motor as a head actuator, the voice coil motor is accelerated to a predetermined speed, and the number of remaining tracks until the target cylinder position is detected during the speed control. When the number of remaining tracks is reduced to a predetermined value, a deceleration current is supplied to the voice coil motor. When the head speed drops to a predetermined speed or less, the control mode is switched to the position control and the head is pulled into the track center, thereby allowing the head to trace the track.
At the same time, the completion of the seeking operation is reported to a higher-order disk control unit. When a head address is designated, the reading or writing operation is executed. On the other hand, on the basis of the designated head address, a position control signal is corrected so as to eliminate the read-out off-track amount with reference to tile correction table in which the off-track amounts which had previously been measured have been stored.
In such a conventional off-track correction, however, after the head address for the reading or writing operation in association with the completion of the seeking operation of the head to the target cylinder position was received, the off-track correction is executed on the basis of the off-track amount read out from the correction table. Therefore, there is a problem such that the off-track correction overlaps the reading or writing operation and the reading or writing operation is waited by a time corresponding to one rotation of the disk in order to perform the off-track correction, so that an access performance deteriorates.
On the other hand, in the measurement of the off-track amount to form the correction table, each of a plurality of magnetic disk units connected manages a time schedule by itself under the control of a magnetic disk control unit, and the off-track measuring process is executed every elapse of a set time.
In the off-track measuring process, the head is sought to the cylinder position on the outside of the magnetic disk data surface where servo information for off-track measurement has been recorded. After completion of the seeking operation, the data heads are sequentially switched, the servo information is read out, and the off-track amount is detected and stored into the correction table in which the head numbers are used as address pointers.
In the conventional off-track measurement as mentioned above, the magnetic disk unit itself executes the off-track measuring process independently of the higher-order disk control unit. There is, consequently, an inconvenience such that the higher-order disk control unit cannot grasp a state of the off-track measuring process in the dependent magnetic disk unit.
That is, after completion of the activation due to the turn-on of the power source of the apparatus, each magnetic disk unit activates a timer which is used for the off-track measurement and executes the off-track measuring process by the interruption each time a preset time elapses. Therefore, there is a case where the off-track measuring process is started due to the interruption during the execution of the command chain from the higher-order apparatus. The execution of the command chain waits until the end of the off-track measuring process and the accessing performance is reduced.
On the other hand, when the disk control unit receives a new command chain from the higher-order apparatus, it is necessary to inquire of the designated magnetic disk unit whether the off-track measuring process is being executed or not. A path is occupied during a period of time for such an inquiry and a frequency of the accessing times to the path increases, so that there is a fear such that a path-busy state frequently occurs.
Further, all of the timers each for detecting the time schedule for the off-track measurement are almost simultaneously started upon completion of the activation after the power source was turned on. However, deviations occur among the counting times of the timers due to various kinds of variations among the magnetic disk units. When the apparatus is used for a long time, the off-track measuring process is executed substantially at random. Therefore, even if the off-track measuring process of a special magnetic disk unit is known by the disk control unit, the operation timings of the other magnetic disk apparatuses are not always set to the off-track measurement timing, it is also difficult to manage in a manner such that by knowing the off-track measurement of one magnetic disk unit, the off-track measurement of the other magnetic disk units is presumed.